Time synchronization system, management node, and time synchronization method

ABSTRACT

A time synchronization system comprises a time master, a time slave, and a management node which are coupled to one another via a plurality of networks. The time master transmits to the time slave a time packet at a given timing through at least one of the plurality of networks. The time slave has a time correcting part for receiving the time packet transmitted from the time master and for correcting time based on the received time packet. The management node has: a network switching determining part for determining whether to switch the network for transmitting a time packet to another of the plurality of networks; and a network switching notifying part for notifying, in a case where it is determined that the network for transmitting the time packet is to be switched, information for identifying the switched-to network to the time slave.

INCORPORATE BY REFERENCE

The present application claims priority from Japanese patent applicationJP 2011-287909 filed on Dec. 28, 2011, the content of which is herebyincorporated by reference into this application.

BACKGROUND OF THE INVENTION

This invention relates to a time synchronization system including a timemaster and a time slave, and more particularly, to a timesynchronization system in which a time slave corrects time managed bythe time slave itself based on a time packet transmitted from a timemaster.

Application fields of information and telecommunication networks areexpanding with the increase in the area and speed of information andtelecommunication networks. Hitherto, systems that utilize ageographically limited information and telecommunication network havebeen built. Today, large-scale systems utilizing a wide area informationand telecommunication network which is not geographically limited arebeing explored. A known example of such systems is energy managementsystems such as a smart grid system.

Large-scale systems that utilize a wide area information andtelecommunication network demand enhanced reliability from the network.A technology for enhancing network reliability that is being exploredinvolves accurately synchronizing time that is managed by a time slavewith time that is managed by a time master by transmitting a time packetfrom the time master through the network.

For instance, IEEE 1588 v2 is a technology for synchronizing time on theorder of microseconds. FIG. 19 is an explanatory diagram of the basicprinciple of IEEE 1588 v2. A time synchronization system in IEEE 1588 v2includes a time master for transmitting a time packet and a time slave,which is coupled to the time master via a network, for receiving thetime packet. The network through which the time packet is communicatedis constructed of a switch that has a transparent clock function. Thetransparent clock function is a function of relaying a packet afterincluding in the packet the length of time required to transmit thepacket within the switch.

FIG. 20 is a sequence diagram illustrating a communication sequenceaccording to the basic time synchronization algorithm of IEEE 1588. Thetime master and the time slave hold two-way communication to synchronizethe time of the time slave with the time of the time master.

The time master periodically transmits a Sync message to the time slave.The time master records a time T1 at which the Sync message has beentransmitted (hereinafter referred to as “Sync transmission time”). Thetime master next transmits a Follow_up message to the time slave. Whentransmitting the Follow_up message, the time master stores the Synctransmission time T1 in the Follow_up message.

The time slave receives the Sync message and, with processing ofreceiving this as a trigger, records a time T2 at which the Sync messagehas been received (hereinafter referred to as “Sync reception time”).The time slave next receives the Follow_up message, and extracts andrecords the Sync transmission time T1 stored in the Follow_up message.The time slave then transmits a Delay_Request message to the timemaster. The time slave records a time T3 at which the Delay_Requestmessage has been transmitted (hereinafter referred to as “Delaytransmission time”).

The time master receives the Delay_Request message and, with processingof receiving this as a trigger, records a time T4 at which theDelay_Request message has been received (hereinafter referred to asDelay reception time”). The time master next transmits a Delay_Responsemessage to the time slave. When transmitting the Delay_Response message,the time master stores the Delay reception time T4 in the Delay_Responsemessage.

The time slave receives the Delay_Response message, and extracts andrecords the Delay reception time T4 stored in the Delay_Responsemessage. Based on the Sync transmission time T1 and the Sync receptiontime T2, the time slave calculates a differential MS_Diff between timeat the time master (hereinafter referred to as “master time”) and timeat the time slave (hereinafter referred to as “slave time”) by thefollowing Expression 1.

MS_Diff=T2−T1  Expression 1

The time slave also obtains a differential between the slave time andthe master time based on the Delay transmission time T3 and the Delayreception time T4 by the following Expression 2.

SM_Diff=T4−T3  Expression 2

A delay in one way between the time master and the time slave (Delay) isobtained next with the use of MS_Diff and SM_Diff and the followingExpression 3.

Delay=(MS_Diff+SM_Diff)/2  Expression 3

The slave time is modified next by obtaining a time offset of the timeslave with respect to the time master (Offset) by the followingExpression 4.

Offset=MS_Diff−Delay=((T2−T1)−(T4−T3))/2  Expression 4

The premise of the calculation method described above is that thenetwork transmission delay time is constant.

On the other hand, when there are fluctuations in transmission delaybetween the time master and the time slave and fluctuations in queuingdelay which occurs in a relay node in the network, a more accuratenetwork transmission delay time needs to be measured in order to obtaina higher synchronization precision. This is why a switch used to build atime synchronization network has a transparent clock function asillustrated in FIG. 21. FIG. 21 is a sequence diagram illustrating acommunication sequence according to a time synchronization algorithm ofa system that includes a switch with a transparent clock function thatconforms to IEEE 1588 v2. The switch with the transparent clock functiontransfers time packets after including the lengths of delays caused inthe switch, namely, a delay time T5−T4, a delay time T7−T6, a delay timeT9−T8, and a delay time T11−T10, in the time packets. This enables thetime slave to take into account the length of delay caused in the switchwhen correcting time.

To build a time synchronization network as this, all switches used tobuild the network need to have a transparent clock function. Mostswitches already disposed in a network, on the other hand, do not have atransparent clock function. Replacing the switches or adding a functionto the switches is therefore required, which increases the cost andlabor of building a time synchronization network. A method aimed tosolve this problem is known which involves having a communication nodemeasure its own delay amount by a simple method and correcting time (seeJP 2011-135482 A, for example).

In another known method for enhancing the precision of timesynchronization, a time slave measures network jitters so that switchesused to build a time synchronization network do not need to have specialmeans and, when the measured jitters are outside a jitter tolerancerange which is set in advance, does not correct time (see JP 2009-077207A, for example).

JP 2009-077207 A also discloses that, when statistical processing ofnetwork jitters measured by the time slave reveals a significantvariation in jitters, switching of networks path used for timesynchronization is detected.

SUMMARY OF THE INVENTION

A problem of the method disclosed in JP 2009-077207 A is that, becausethe time slave needs to receive a time packet a plurality of timesbefore detecting switching of network paths and cannot detect switchingof network paths immediately after the switching, the precision of timesynchronization drops for that duration.

Another problem of the method of JP 2009-077207 A is that switching ofnetwork paths cannot be detected unless jitters vary greatly between theswitched-from network path and the switched-to network path.

An object of this invention to provide a time synchronization systemthat uses a plurality of networks that executes an appropriate timecorrection even immediately after switching of networks by minimizingthe time required for a time slave to detect switching of networks.

According to an aspect of the present invention, there is provided atime synchronization system, comprising a time master, a time slave, anda management node which are coupled to one another via a plurality ofnetworks, wherein the time master transmits to the time slave a timepacket at a given timing through at least one of the plurality ofnetworks, wherein the time slave has a time correcting part forreceiving, through the at least one of the plurality of networks, thetime packet transmitted from the time master and for correcting timebased on the received time packet, and wherein the management node has:a network switching determining part for determining whether to switchthe network for transmitting a time packet to another of the pluralityof networks; and a network switching notifying part for notifying, in acase where it is determined that the network for transmitting the timepacket is to be switched, information for identifying the switched-tonetwork to the time slave.

An effect of the exemplary embodiment of the invention disclosed hereinis summarized as follows. This invention enables to provide a timesynchronization system that uses a plurality of networks that executesan appropriate time correction even immediately after switching ofnetworks by minimizing the time required for a time slave to detectswitching of networks.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be appreciated by the description whichfollows in conjunction with the following figures, wherein:

FIG. 1 is an explanatory diagram of a configuration of a timesynchronization system according to a first embodiment of thisinvention;

FIG. 2 is an explanatory diagram of a configuration of each time slaveaccording to the first embodiment of this invention;

FIG. 3 is an explanatory diagram of a configuration of a time correctingpart according to the first embodiment of this invention;

FIG. 4 is an explanatory diagram of a threshold table according to thefirst embodiment of this invention;

FIG. 5 is an explanatory diagram of a modification example of thethreshold table according to the first embodiment of this invention;

FIG. 6 is an explanatory diagram of an operation value table accordingto the first embodiment of this invention;

FIG. 7 is a flow chart of time correcting processing that is executed bythe time correcting part according to the first embodiment of thisinvention;

FIG. 8 is a sequence diagram in which a management node sends a usednetwork switching instruction to one of the time slaves according to thefirst embodiment of this invention;

FIG. 9 is a sequence diagram of processing of reading the operationvalue table and processing of rewriting the threshold table which areexecuted by the management node according to the first embodiment ofthis invention;

FIG. 10 is a flow chart of threshold change determining processingaccording to the first embodiment of this invention;

FIG. 11 is an explanatory diagram of a configuration of the managementnode according to the first embodiment of this invention;

FIG. 12 is a flow chart of presetting processing which is executed toset the threshold table in advance by a table control part of the timecorrecting part according to the first embodiment of this invention;

FIG. 13 is a flow chart of processing of rewriting a utilization statusin the threshold table which is executed by the table control part ofthe time correcting part according to the first embodiment of thisinvention;

FIG. 14 is an explanatory diagram of the configuration of the timecorrecting part according to a second embodiment of this invention;

FIG. 15 is a flow chart of the time correcting processing that isexecuted by the time correcting part according to the second embodimentof this invention;

FIG. 16 is an explanatory diagram of the configuration of the timesynchronization system according to a third embodiment of thisinvention;

FIG. 17 is an explanatory diagram of a configuration of a networkapparatus according to the third embodiment of this invention;

FIG. 18 is an explanatory diagram of the configuration of the timecorrecting part according to the third embodiment of this invention;

FIG. 19 is an explanatory diagram of a basic principle of IEEE 1588 v2;

FIG. 20 is a sequence diagram illustrating a communication sequenceaccording to a basic time synchronization algorithm of IEEE 1588; and

FIG. 21 is a sequence diagram illustrating a communication sequenceaccording to a time synchronization algorithm of a system that includesa switch with a transparent clock function that conforms to IEEE 1588v2.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment

A first embodiment of this invention is described below with referenceto FIGS. 1 to 13.

A time synchronization system of this embodiment is a timesynchronization system utilizing a wide area network that couples aplurality of bases to one another. The time synchronization systemincludes a management node for transmitting a network switchingnotification to a time slave when a network to which a time mastertransmits a time packet is switched to another network. This enables thetime slave to quickly detect that networks have been switched.

FIG. 1 is an explanatory diagram of the configuration of the timesynchronization system according to the first embodiment of thisinvention.

In the time synchronization system, a control base 400 where a timemaster 10 is disposed is coupled to bases 406 to 408 where time slaves13 are disposed by wide area networks 405A and 405B (hereinafter thewide area networks 405A and 405B are collectively referred to as widearea networks 405).

The time master 10, an application server 401, a management node 402,and a network node 404 are disposed in the control base 400. The networknode 404 is coupled to the wide area networks 405. The time master 10,the application server 401, the management node 402, and the networknode 404 are coupled to one another via an intra-base network 403.

The time master 10 periodically transmits a time packet to the timeslaves 13 through the wide area networks 405. The application server 401is a computer for executing an application. The management node 402 is acomputer for managing the wide area networks 405 which are used by thetime master 10 to transmit a time packet. The network node 404 is arouter or a switch that has a communication function.

The application server 401 collects, via the time slaves 13 and the widearea networks 405, various types of data detected by sensors 409, whichare described later. The application server 401 saves the collectedvarious types of data, or executes computing processing based on thevarious types of data.

One of the time slaves 13, one of the sensors 409, and a controlapparatus 410 are disposed in the base 406. The sensor 409 and thecontrol apparatus 410 are connected to the time slave 13 by a wired orwireless communication line.

The time slave 13 is a network node coupled to one of the wide areanetworks 405, and is a sophisticated network node which has adetermining processing function and a computing processing function inaddition to a communication function. The time slave 13 has a functionof counting time, and synchronizes in time with the time master 10 byreceiving a time packet from the time master 10. The time slave 13 alsomonitors data detected by the sensor 409, and executes computingprocessing on the data. The time slave 13 further executes processingrelated to the control apparatus 410.

A time packet that the time slave 13 receives is delayed by atransmission delay time of the wide area network 405 that is used forthe transmission of the time packet. The transmission delay time varieseach time the time slave 13 receives a time packet. The variations intransmission delay time are called transmission delay time fluctuations(jitters). The transmission delay time fluctuations cause a timesynchronization error for the time slave 13. In other words, when thetransmission delay time of a time packet is too great, the time slave 13cannot synchronize with the time of the time master 10 in some casesdespite correcting time based on the time packet.

A jitter threshold for determining whether to correct time is thereforeset to the time slave 13 of this embodiment for each wide area network405. When jitters of a time packet are within the threshold, the timeslave 13 corrects time based on the time packet.

In this embodiment, the time slave 13 manages the threshold for eachwide area network 405 used for the transmission of a time packet andchanges, based on the used network switching notification which is sentfrom the management node 402, the threshold used to determine whether tocorrect time. Whether to correct time can thus be determined with theuse of a threshold associated with the switched-to wide area network405, even immediately after the wide area networks 405 are switched.

In this embodiment, the time slave 13 stores an operation value relatedto a time packet that has actually been received by the time slave 13,and the management node 42 reads the operation value to determinewhether or not the threshold is appropriate based on the read operationvalue. The propriety of the threshold can thus be examined.

The sensor 409 and the control apparatus 410 are apparatus that utilizetime counted by the time slave 13. The sensor 409 refers collectively tovarious mechanisms for converting temperature, voltage value, and othertypes of physical information into a signal, such as a temperaturesensor, a humidity sensor, a vibration sensor, a pressure sensor, ahuman presence sensor, a microphone, and a camera. The control apparatus410 refers collectively to various mechanisms for converting an inputsignal into physical movement, such as an alarm apparatus, an openingand closing switch, and a motor.

The time slave 13 does not always need to be connected to the sensor 409and the control apparatus 410, and it is sufficient if some apparatusthat utilizes time counted by the time slave 13 is connected to the timeslave 13.

The time slave 13 is disposed in each of the bases 406 to 408 in thedescription above. Instead of disposing one time slave 13 in the bases406 to 408 each, a room-by-room basis, a floor-by-floor basis, or abuilding-by-building basis placement may be employed depending on theprocessing performance of the time slave 13.

The counts of the time slaves 13, the sensors 409, the control apparatus410, and the wide area networks 405 are not limited to those in FIG. 1.

FIG. 2 is an explanatory diagram of the configuration of each time slave13 according to the first embodiment of this invention.

The time slave 13 includes hardware 54 which includes a centralprocessing unit (CPU) 55, a memory 56, an input/output unit (I/O) 57,and others. The CPU 55 is a processing part for executing various typesof computing processing. The memory 56 is a storage part for storingvarious programs and data. The I/O 57 is an interface to the wide areanetworks 405, the relevant sensor 409, and the relevant controlapparatus 409.

An operating system (OS) 53 resides in a layer above the hardware 54.The OS 53 is executed by the CPU 55.

A time correcting part 50, a time counting part 51, and an application52 are located in a layer above the OS 53. The time correcting part 50,the time counting part 51, and the application 52 are implemented by theCPU 55 by executing their corresponding programs.

The time correcting part 50 corrects time that the time counting part 51counts, based on a time packet received by the time slave 13. The timecounting part 51 counts time. The application 52 executes processingrelated to the sensor 409 and the control apparatus 410 that areconnected to the time slave 13.

FIG. 3 is an explanatory diagram of the configuration of the timecorrecting part 50 according to the first embodiment of this invention.

The time correcting part 50 includes packet transmitting/receiving parts604 to 606, a time packet transmitting/receiving part 607, a jittercalculating part 608, an operation value table 609, a time correctionexecution feasibility determining part 610, a threshold table 611, and atable control part 612.

The packet transmitting/receiving parts 604 to 606 are connected tocommunication interfaces 601 to 603 to transmit/receive packets. Thepacket transmitting/receiving part 604 is connected to the communicationinterface 601, which is coupled to a communication path 613 (the widearea network 405A). The packet transmitting/receiving part 605 isconnected to the communication interface 602, which is coupled to acommunication path 614 (the wide area network 405B). The packettransmitting/receiving part 606 is connected to the communicationinterface 603, which is coupled to a communication path 615 (a networkcoupled to the management node 402).

A time packet that is transmitted to the wide area network 405A from thetime master 10 is received by the packet transmitting/receiving part 604through the communication path 613 and the communication interface 601.A time packet that is transmitted to the wide area network 405B from thetime master 10 is received by the packet transmitting/receiving part 605through the communication path 614 and the communication interface 602.

In the manner described above, this embodiment is configured so that onepacket communication path for transmitting a time packet, onecommunication interface, and one packet transmitting/receiving part areprovided for each wide area network 405 that is used for time packettransmission.

The time packet transmitting/receiving part 607 receives a time packetwhen the packet transmitting/receiving part 604 or the packettransmitting/receiving part 605 inputs a time packet to the time packettransmitting/receiving part 607. The time packet transmitting/receivingpart 607 is connected to the jitter calculating part 608 and the timecorrection execution feasibility determining part 610.

The jitter calculating part 608 is connected to the operation valuetable 609 and the time correction execution feasibility determining part610, and calculates jitters of a time packet received by the time packettransmitting/receiving part 607 that are due to the relevant wide areanetwork 405. Details of how the jitters are calculated are describedwith reference to FIG. 7.

The operation value table 609 is connected to the table control part612. Registered in the operation value table 609 are values that arerelated to a received time packet and that are used in threshold changedetermining processing illustrated in FIG. 10, which is executed by themanagement node 402. Details of the operation value table 609 aredescribed with reference to FIG. 6.

The time correction execution feasibility determining part 610 isconnected to the operation value table 609 and the threshold table 611,and refers to the threshold table 611 to determine whether to executeprocessing of correcting time based on a received time packet. When itis determined that the time correcting processing is to be executed, thetime correction execution feasibility determining part 610 corrects timeand outputs corrected time information to the time counting part 51through a communication path 616.

The threshold table 611 is connected to the table control part 612.Values used in the determining processing that is executed by the timecorrection execution feasibility determining part 610 are registered inthe threshold table 611. Details of the threshold table 611 aredescribed with reference to FIGS. 4 and 5.

The table control part 612 controls the operation value table 609 andthe threshold table 611.

FIG. 4 is an explanatory diagram of the threshold table 611 according tothe first embodiment of this invention. The threshold table 611 isstored in the memory 56.

The threshold table 611 includes a network name 71, a utilization status72, a delay median 73, a jitter median 74, and a jitter tolerance 75.

Information by which the wide area network 405 that is coupled to thetime slave 13 can be identified is registered as the network name 71.Information for identifying which wide area network 405 is being usedfor time packet transmission is registered as the utilization status 72.A median of the lengths of time packet delays that are due to the widearea network 405 is registered as the delay median 73. A median ofjitters of time packets that are due to the wide area network 405 isregistered as the jitter median 74. Registered as the jitter tolerance75 is a value used in the determining processing that is executed by thetime correction execution feasibility determining part 610 and thatindicates a range set around the registered value of the jitter median74.

Values registered in the threshold table 611 can have an arbitraryprecision and unit, which are decided when the time synchronizationsystem is built.

FIG. 5 is an explanatory diagram of a modification example of thethreshold table 611 according to the first embodiment of this invention.

The threshold table 611 of FIG. 5 differs from the threshold table 611of FIG. 4 in that a minimum jitter value 76 and a maximum jitter value77 are included instead of the jitter median 74 and the jitter tolerance75.

A minimum value of jitters used in the determining processing that isexecuted by the time correction execution feasibility determining part610 is registered as the minimum jitter value 76. A maximum value ofjitters used in the determining processing that is executed by the timecorrection execution feasibility determining part 610 is registered asthe maximum jitter value 77.

FIG. 6 is an explanatory diagram of the operation value table 609according to the first embodiment of this invention.

The operation value table 609 includes a network name 81, a mean delay82, a mean jitter 83, a jitter standard deviation 84, a time packetarrival count 85, and a time correction cancellation count 86.

Information by which the wide area network 405 that is coupled to thetime slave 13 can be identified is registered as the network name 81. Amean value of the lengths of time packet delays that are due to the widearea network 405 is registered as the mean delay 82. A mean value ofjitters of time packets that are due to the wide area network 405 isregistered as the mean jitter 83. A standard deviation of jitters oftime packets that are due to the wide area network 405 is registered asthe jitter standard deviation 84. The number of times the time slave 13has received a time packet is registered as the time packet arrivalcount 85. The number of times the time correction execution feasibilitydetermining part 610 has determined the time correcting processing isnot to be executed is registered as the time correction cancellationcount 86.

The mean delay 82, the mean jitter 83, and the jitter standard deviation84 are values related to a delay of a time packet that is due to a widearea network. The jitter standard deviation 84 may not necessarily beincluded in the operation value table 609.

Values registered in the operation value table 609 can have an arbitraryprecision and unit, which are decided when the time synchronizationsystem is built.

FIG. 7 is a flow chart of the time correcting processing that isexecuted by the time correcting part 50 according to the firstembodiment of this invention.

First, the time packet transmitting/receiving part 607 receives a Syncmessage contained in a time packet (101). The time packettransmitting/receiving part 607 next receives a Follow_up messagecontained in the time packet (102). The time packettransmitting/receiving part 607 then transmits a Delay_Req message tothe time master 10 (103). Next, the time packet transmitting/receivingpart 607 receives a Delay_Resp message out of a time packet (104).

Steps 101 to 104 are processing defined in IEEE 1588 v2, and the timeslave 13 can obtain a time packet by executing these processing steps.

Next, the jitter calculating part 608 calculates the amount of delay ofthe received time packet that is due to the wide area network 405, andcompares the calculated delay amount with a value registered as thedelay median 73 in the threshold table 611 to calculate jitters (105).Specifically, the jitter calculating part 608 calculates the delayamount of the wide area network 405 with the use of the times T1, T2,T3, and T4 in the time synchronization method according to IEEE 1588 v2which has been described with reference to FIG. 20. The delay amount ofthe wide area network 405 is calculated by Expression 5.

Delay amount={(T4−T3)+(T2−T1)}/2  Expression 5

Jitters are calculated by Expression 6.

Jitter=delay median−delay amount  Expression 6

In Step 105, the jitter calculating part 608 also updates valuesregistered as the mean delay 82, the mean jitter 83, and the jitterstandard deviation 84 in the operation value table 609 with the use ofthe calculated delay amount and jitters. The jitter calculating part 608increments the time packet arrival count 85 of the operation value table609 as well.

Next, the time correction execution feasibility determining part 610reads information registered as the utilization status 72 in thethreshold table 611 to determine the wide area network 405 that has beenused in the transmission of the received time packet (106).

The time correction execution feasibility determining part 610 nextsearches entries of the threshold table 611 for an entry for the widearea network 405 that has been used in the transmission of the receivedtime packet, and reads, as a threshold, values that are registered inthe entry as the jitter median 74 and the jitter tolerance 75 (107).

The time correction execution feasibility determining part 610 comparesthe jitters calculated in Step 105 and the threshold read in Step 107 todetermine whether or not the jitters calculated in Step 105 are smallerthan the threshold read in Step 107 (108).

When it is determined in Step 108 that the jitters calculated in Step105 are smaller than the threshold read in Step 107, the time correctionexecution feasibility determining part 610 executes processing ofcorrecting time based on the received time packet (109), outputscorrected time information to the time counting part 51, and ends thetime correcting processing.

When it is determined in Step 108 that the jitters calculated in Step105 are equal to or more than the threshold read in Step 107, the timecorrection execution feasibility determining part 610 increments thetime correction cancellation count 86 of the operation value table 609and ends the time correcting processing, without executing thecorrecting processing of Step 109 which is processing of correcting timebased on the received time packet.

FIG. 8 is a sequence diagram in which the management node 402 sends aused network switching instruction to one of the time slaves 13according to the first embodiment of this invention.

In the case where the time synchronization system has its own wide areanetworks 405, or in similar cases, communication equipment 111 used tobuild one of the wide area networks 405 detects a failure (113A) andtransmits failure information which indicates the occurrence of afailure to the management node 402 (114A).

In the case where users use the wide area networks 405 provided by awide area network service provider 112, or in similar cases, a server ofthe wide area network service provider 112 detects a failure (113B) andtransmits failure information which indicates the occurrence of afailure to the management node 402 (114B).

Alternatively, the management node 402 may detect a failure by readingfailure information out of the communication equipment 111 or the serverof the wide area network service provider 112.

The management node 402 receives the failure information from thecommunication equipment 111 or the server of the wide area networkservice provider 112, and executes network switching determiningprocessing (115A and 115B). The network switching determining processingis processing that is executed when a failure is detected in the widearea network 405 that has been used for time packet transmission inorder to identify the wide area network 405 where a failure has not beendetected as the wide area network 405 to be newly used for time packettransmission (a switched-to wide area network).

The management node 402 next transmits to the network node 404 a usednetwork switching instruction that includes information for identifyingthe switched-to wide area network (116A and 116B), and transmits theused network switching instruction to the relevant time slave 13 (117Aand 117B).

The network node 404 receives the used network switching instruction andswitches the wide area network 405 for transmitting a packet that isreceived from the intra-base network 403 to the switched-to wide areanetwork 405. The wide area network 405 to which the time mastertransmits a time packet is thus switched to the switched-to wide areanetwork.

The time slave 13 that receives the used network switching instructionrewrites the utilization status 72 in the threshold table 611 based onthe used network switching instruction received. Details of thisprocessing are described with reference to FIG. 13.

FIG. 9 is a sequence diagram of processing of reading the operationvalue table 609 and processing of rewriting the threshold table 611which are executed by the management node 402 according to the firstembodiment of this invention.

The management node 402 starts reading, at a given timing, operationvalues registered in the operation value table 609 that is stored in thetime slave 13 of interest (121), and transmits an operation valuereading command to the time slave 13 (122). The operation value readingcommand includes information for identifying the wide area network 405of which operation values are to be read.

The time slave 13 receives the operation value reading command andselects, from among entries of the operation value table 609, an entrywhere the network name 81 matches the information for identifying one ofthe wide area networks 405 which is included in the received operationvalue reading command. The time slave 13 reads, as operation values,values registered in the selected entry as the mean delay 82, the meanjitter 83, the jitter standard deviation 84, the time packet arrivalcount 85, and the time correction cancellation count 86 (123). The timeslave 13 transmits the read operation values to the management node 402(124).

The management node 402 receives the operation values and executesthreshold change determining processing for determining whether or notthe threshold needs to be changed based on the received operation values(125). Details of the threshold change determining processing aredescribed with reference to FIG. 10.

When it is determined by the threshold change determining processingthat the threshold needs to be changed, the management node 402transmits to the time slave 13 a threshold rewrite command which is acommand to rewrite a threshold registered in the threshold table 611 ofthe time slave 13 (126). The threshold rewrite command includesinformation for identifying the wide area network 405 for which thethreshold is to be rewritten and a value with which the threshold isrewritten (a threshold rewrite value).

The time slave 13 receives the threshold rewrite command and, based onthe received threshold rewrite command, rewrites the thresholdregistered in the threshold table 611 (127). Specifically, the timeslave 13 selects from among entries of the threshold table 611 an entrywhere the network name 71 matches the information for identifying one ofthe wide area networks 405 which is included in the threshold rewritecommand. The time slave 13 rewrites the threshold in the selected entry.In the case where the threshold table 611 stored in the time slave 13 isas shown in FIG. 4, the time slave 13 rewrites the jitter tolerance 75based on the threshold rewrite value. In the case where the thresholdtable 611 stored in the time slave 13 is as illustrated in FIG. 5, onthe other hand, the time slave 13 rewrites the minimum jitter value 76and the maximum jitter value 77 each with the threshold rewrite value.

After rewriting the threshold, the time slave 13 transmits a rewritecompletion notification to the management node 402 (128).

In the manner described above, the management node 402 determineswhether or not the currently set threshold is appropriate based oninformation about a time packet that the time slave 13 has actuallyreceived and, when the threshold is not appropriate, changes thethreshold. The threshold can thus be kept appropriate all the time.

The management node 402 in the description given above includes, in anoperation value reading command, information for identifying the widearea network 405 of which operation values are to be read.Alternatively, all entries where operation values of the wide areanetworks 405 are registered may be read out of the operation value table609 that is stored in the time slave 13 of interest. For example, themanagement node 402 may transmit to the time slave 13 an operation valuereading command that includes information indicating the reading of allentries of the operation value table 609. Then the time slave 13 reads,as operation values, values registered as the mean delay 82, the meanjitter 83, the jitter standard deviation 84, the time packet arrivalcount 85, and the time correction cancellation count 86 out of allentries, and transmits the read operation values to the management node402. The management node 402 determines for each read entry whether ornot the threshold needs to be changed, and transmits threshold rewritecommands only for entries where the threshold needs to be changed.

In FIG. 9, the management node 402 transmits an operation value readingcommand to the time slave 13 at a given timing. Alternatively, the timeslave 13 may notify the management node 402 when an operation valueregistered in the operation value table 609 satisfies a given conditionof the fact so that the management node 402 notified of the fact thatthe operation value satisfies a given condition transmits an operationvalue reading command to the time slave 13. For example, when the jitterstandard deviation 84 of the operation value table 609 exceeds a givenvalue “1.5”, the time slave 13 may notify the fact.

This ensures that the threshold change determining processing isexecuted only the likelihood of the threshold not being appropriate isstrong, and thus lessens processing load on the management node 402.

FIG. 10 is a flow chart of the threshold change determining processingaccording to the first embodiment of this invention.

The threshold change determining processing is processing included in amanagement application 131, which is illustrated in FIG. 11, and isexecuted by a CPU 134 of FIG. 11.

The management node 402 first determines whether or not a valueregistered as the time correction cancellation count 86 that is includedamong the received operation values is within a given range (2101).

When it is determined that the value registered as the time correctioncancellation count 86 is not within the given range, the management node402 determines that the threshold needs to be changed (2104), determinesa value with which the threshold is to be rewritten, and ends thethreshold change determining processing.

Specifically, in the case where the value registered as the timecorrection cancellation count 86 is lower than the lower limit value ofthe given range, it means that fluctuations in time packet delay due tothe relevant wide area network 405 are small, and the management node402 accordingly determines a rewrite value that narrows the thresholdrange by a given amount. In the case where the value registered as thetime correction cancellation count 86 is higher than the upper limitvalue of the given range, on the other hand, it means that fluctuationsin time packet delay due to the relevant wide area network 405 arelarge, and the management node 402 accordingly determines a rewritevalue that widens the threshold range by a given amount. The givenamount by which the threshold range is changed can be set by a user.

When it is determined that the value registered as the time correctioncancellation count 86 is within the given range, the management node 402calculates (a value registered as the mean jitter 83)±(a valueregistered as the jitter standard deviation 84), and determines whetheror not the calculated value is within a given range (2102).

When it is determined that the value calculated in Step 2102 is notwithin the given range, the management node 402 proceeds to Step 2104 todetermine a threshold rewrite value, and then ends the threshold changedetermining processing. That the value calculated in Step 2102 isoutside the given range signifies here large fluctuations in time packetdelay due to the relevant wide area network 405, and the management node402 accordingly determines a rewrite value that widens the thresholdrange by a given amount. When it is determined that the value calculatedin Step 2102 is within the given range, the management node 402 maydetermine that the threshold does not need to be changed, and ends thethreshold change determining processing.

When it is determined that the value calculated in Step 2102 is withinthe given range, the management node 402 determines whether or not avalue registered as the mean delay 82 that is included among thereceived operation values is within a given range (2103). It ispreferred to set as the given range some range centered around the delaymedian 73 that is associated with this mean delay 82.

When it is determined in Step 2103 that the value registered as the meandelay 82 is not within the given range, the management node 402 proceedsto Step 2104 to determine a threshold rewrite value, and then ends thethreshold change determining processing. That the value registered asthe mean delay 82 is outside the given range signifies here largefluctuations in time packet delay due to the relevant wide area network405, and the management node 402 accordingly determines a rewrite valuethat widens the threshold range by a given amount.

When it is determined in Step 2103 that the value registered as the meandelay 82 is within the given range, the management node 402 determinesthat the threshold does not need to be changed, and ends the thresholdchange determining processing.

The threshold change determining processing may instead be executed bythe time slave 13 by referring to the operation value table 609. Thetime slave 13 in this case notifies the result of the determiningprocessing to the management node 402.

This invention encompasses various modification examples of Step 2102where the management node 402 calculates (mean jitter)±(jitter standarddeviation). For instance, the management node 402 may calculate (meanjitter)±2×(jitter standard deviation). This is because Step 2102 is fordetermining the range and distribution of jitters.

Step 2101 where the management node 402 determines whether or not avalue registered as the time correction cancellation count 86 that isincluded among the received operation values is within a given range mayalso be modified so that the management node 402 determines whether ornot the ratio of a value registered as the time correction cancellationcount 86 that is included among the received operation values to a valueregistered as the time packet arrival count 85 that is included amongthe received operation values is within a given range.

FIG. 11 is an explanatory diagram of the configuration of the managementnode 402 according to the first embodiment of this invention.

The management node 402 has hardware 133 which includes the centralprocessing unit (CPU) 134, a memory 135, an input/output unit (I/O) 136,and others. The CPU 134 is a processing part for executing various typesof computing processing. The memory 135 is a storage part for storingvarious programs and data. The I/O 136 is an interface to the intra-basenetwork 403.

An operating system (OS) 132 resides in a layer above the hardware 133.The OS 132 is executed by the CPU 134.

The management application 131 is located in a layer above the OS 132.The management application 131 is implemented by the CPU 134 byexecuting a corresponding program.

The management application 131 includes the network switchingdetermining processing which is described above with reference to FIG. 8and denoted by 115A and 115B, the operation value reading startprocessing 121 which is described above with reference to FIG. 9, andthe threshold change determining processing 125 which is described abovewith reference to FIGS. 9 and 10.

FIG. 12 is a flow chart of presetting processing which is executed toset the threshold table 611 in advance by the table control part 612 ofthe time correcting part 50 according to the first embodiment of thisinvention.

The table control part 612 first receives pre-settings information ofthe threshold table 611 from the management node 402 (141). Thepre-settings information includes the name and utilization status of oneof the wide area networks 405, a delay median, a jitter median, and ajitter tolerance.

The table control part 612 next writes the received pre-settingsinformation in the threshold table 611 (142).

Specifically, the table control part 612 creates a new entry to thethreshold table 611 to write, in the created entry, the name of the widearea network 405 that is included in the pre-settings information as thenetwork name 71, the utilization status that is included in thepre-settings information as the utilization status 72, the delay medianthat is included in the pre-settings information as the delay median 73,the jitter median that is included in the pre-settings information asthe jitter median 74, and the jitter tolerance that is included in thepre-settings information as the jitter tolerance 75.

The table control part 612 then notifies the management node 402 of thecompletion of the write in the threshold table 611 (143), and ends thepresetting processing.

The threshold table 611 is thus a table set in advance, and themanagement node 402 can set the threshold table 611 in advance.

FIG. 13 is a flow chart of processing of rewriting a utilization statusin the threshold table 611 which is executed by the table control part612 of the time correcting part 50 according to the first embodiment ofthis invention. The utilization status rewriting processing is executedrepeatedly at a given timing.

The table control part 612 first determines whether or not a usednetwork switching instruction transmitted from the management node 402has been received (151).

When it is determined in Step 151 that a used network switchinginstruction has not been received, the table control part 612 ends theutilization status rewriting processing.

When it is determined in Step 151 that a used network switchinginstruction has been received, on the other hand, the table control part612 rewrites the utilization status 72 of the threshold table 611 basedon the used network switching instruction (152), and then ends theutilization status rewriting processing.

Specifically, the table control part 612 selects from among entries ofthe threshold table 611 an entry where the network name 71 matchesinformation that is included in the used network switching instructionreceived and that identifies the switched-to wide area network 405, andupdates the utilization status 72 to “in use” in the selected entry. Thetime slave 13 also selects from among entries of the threshold table 611an entry where the utilization status 72 has been “in use” and updatesthe utilization status to “not in use” in the selected entry.

In this way, the time correction execution feasibility determining part610 of the time slave 13 can complete processing of reading a thresholdout of the threshold table 611 (Step 107 of FIG. 7) more quickly than inthe case where the wide area network 405 through which a time packet isreceived is identified each time.

In this embodiment, the management node 402 manages which wide areanetwork 405 is used by the time master 10 to transmit a time packet and,when the wide area network 405 used by the time master 10 for timepacket transmission is switched to another wide area network 405,notifies the switched-to wide area network 405 to the relevant timeslave 13. The time slave 13 can thus detect the switching of the widearea network 405 used by the time master 10 for time packet transmissionimmediately after the switching.

The management node 402 also determines for each time slave 13 thepropriety of the threshold based on the value related to a time packetthat has actually been received by the time slave 13, thereby enablingthe time slave 13 to base the determination of whether to execute timecorrection on an appropriate threshold.

Second Embodiment

A second embodiment of this invention is described below with referenceto FIGS. 14 and 15.

In the first embodiment, a case where the time master 10 selects anduses one out of a plurality of wide area networks 405 to transmit a timepacket has been described. Described in the second embodiment is thedifference from the first embodiment of a case where the time master 10selects and uses at least one out of a plurality of wide area networks405 to transmit a time packet.

FIG. 14 is an explanatory diagram of the configuration of a timecorrecting part 1600 according to the second embodiment of thisinvention. Components of the time correcting part 1600 that are the sameas those of the time correcting part 50 illustrated in FIG. 3 aredenoted by the same reference symbols, and descriptions thereof areomitted.

The time correcting part 1600 of this embodiment differs from the timecorrecting part 50 of the first embodiment in that one time packettransmitting/receiving part 607 and one jitter calculating part 608 areprovided for each wide area network 405. Specifically, a time packettransmitting/receiving part A 607A is connected to the packettransmitting/receiving part 604, which is coupled to the wide areanetwork 405A, and a jitter calculating part A 608A is connected to thetime packet transmitting/receiving part A 607A. A time packettransmitting/receiving part B 607B is connected to the packettransmitting/receiving part 605, which is coupled to the wide areanetwork 405B, and a jitter calculating part B 608B is connected to thetime packet transmitting/receiving part B 607B.

FIG. 15 is a flow chart of the time correcting processing that isexecuted by the time correcting part 1600 according to the secondembodiment of this invention. Steps of the time correcting processing ofthis embodiment that are the same as those of the time correctingprocessing of the first embodiment illustrated in FIG. 7 are denoted bythe same reference symbols, and descriptions thereof are omitted.

In the time correcting processing of this embodiment, when it isdetermined in Step 108 that the jitters calculated in Step 105 aresmaller than the threshold read in Step 107, the time correctionexecution feasibility determining part 610 determines whether or not thejitters calculated in Step 105 are the smallest of jitters of the widearea networks 405 that are in use (1701).

When it is determined in Step 1701 that the jitters calculated in Step105 are the smallest of jitters of the wide area networks 405 that arein use, the time correction execution feasibility determining part 610proceeds to Step 109 to execute time correction processing based on thereceived time packet (109), outputs corrected time information to thetime counting part 51, and ends the time correcting processing.

When it is determined in Step 1701 that the jitters calculated in Step105 are not the smallest of jitters of the wide area networks 405 thatare in use, on the other hand, the time correction execution feasibilitydetermining part 610 increments the time correction cancellation count86 of the operation value table 609 and ends the time correctingprocessing, without executing the processing (109) of correcting timebased on the received time packet.

In this embodiment, where the time master 10 uses a plurality of widearea networks 405 to transmit a time packet, the time correcting part1600 receives time packets from a plurality of wide area networks 405.Jitters therefore vary depending on through which wide area network 405the time packet has been received. Consequently, if the time correctingprocessing is executed without executing Step 1701, there is a chancethat the correction makes time inaccurate. This invention prevents timeinaccuracy due to correction by executing time correcting processingonly when it is confirmed in Step 1701 that the jitters calculated inStep 105 are the smallest of jitters of the wide area networks 405 thatare in use.

Third Embodiment

A third embodiment of this invention is described below with referenceto FIGS. 16 to 18.

In this embodiment, a network apparatus 181 is provided for each timeslave 13 to be placed between the time slave 13 and the wide areanetworks 405. The difference of the third embodiment from the firstembodiment is described below.

Each time slave 13 in the first embodiment has a communication interfaceand a packet transmitting/receiving part for each of the wide areanetworks 405 and the network that is coupled to the management node (thecommunication interfaces 601 to 603 and the packettransmitting/receiving part 604 to 606). In this embodiment, the networkapparatus 181 converts three physical network interfaces into onenetwork interface so that the time slave 13 only needs to have onecommunication interface 2001 illustrated in FIG. 18 and one packettransmitting/receiving part 2002 illustrated in FIG. 18.

FIG. 16 is an explanatory diagram of the configuration of the timesynchronization system according to the third embodiment of thisinvention.

The network apparatus 181 is coupled to the wide area networks 405A and405B, and connected to the time slave 13 as well. In other words, thenetwork apparatus 181 relays between the wide area networks 405 and thetime slave 13.

FIG. 17 is an explanatory diagram of the configuration of the networkapparatus 181 according to the third embodiment of this invention.

The network apparatus 181 is coupled to the wide area networks 405 sidevia communication paths 1912 to 1914, and is connected to the time slave13 via a communication path 1911.

The network apparatus 181 has three communication interfaces 1902 to1904 and three packet transmitting/receiving parts 1905 to 1907 on thewide area network 405 side, and one communication interface 1910 and onepacket transmitting/receiving part 1909 on the time slave 13 side.

In the network apparatus 181, the packet transmitting/receiving parts1905 to 1907 on the wide area network 405 side and the packettransmitting/receiving part 1909 on the time slave 13 side are connectedto each other via an exclusive access control/deduplication part 1908.

The exclusive access control/deduplication part 1908 has a function ofperforming exclusive access control over a plurality of received packetsand thereby communicating a desired packet which is selected from amongthe plurality of packets, and a function of performing deduplication ona plurality of packets and thereby communicating a plurality of receivedpackets from which duplicate packets have been removed.

FIG. 18 is an explanatory diagram of the configuration of a timecorrecting part 2000 according to the third embodiment of thisinvention. Components of the time correcting part 2000 of thisembodiment that are the same as those of the time correcting part 50 ofthe first embodiment illustrated in FIG. 3 and the time correcting part1600 of the second embodiment illustrated in FIG. 14 are denoted by thesame reference symbols, and descriptions thereof are omitted.

The time slave 13 has the communication interface 2001, which isconnected to a communication path 2010 leading to the network apparatus181. The time correcting part 2000 includes the packettransmitting/receiving part 2002, which is connected to thecommunication interface 2001 to transmit/receive packets to/from thenetwork apparatus 181.

This embodiment has a feature in that one communication path 2010, onecommunication interface 2001, and one packet transmitting/receiving part2002 are provided, as opposed to a plurality of communication paths2010, a plurality of communication interfaces 2001, and a plurality ofpacket transmitting/receiving parts 2002. The communication interface2001 communicates information to/from the time master 10 and themanagement node 402. The packet transmitting/receiving part 2002 passesa received packet to the time packet transmitting/receiving part 607when the received packet is information from the time master 10, andpasses a received packet to the table control part 612 when the receivedpacket is information from the management node 402.

The packet transmitting/receiving part 2002 can identify from whichnetwork a packet has been received because the communication interfaces1902 to 1904 or packet transmitting/receiving parts 1905 to 1907 of thenetwork apparatus 181 assign their own identifiers to received packets,and the packet transmitting/receiving part 1909 and the communicationinterface 1910 transmit the packets with the identifiers assigned.

The rest of the configuration of the time correcting part 2000 is thesame as that of the time correcting part 50 of the first embodiment andthe time correcting part 1600 of the second embodiment.

As has been described, according to the present invention, the timeslave 13 can detect in a reduced time switching of the wide area network405 that is used by the time master 10 for the transmission of a timepacket, and a right determination can be made about whether to correcttime even immediately after the switching of the wide area network 405.

This invention is optimum for, for example, a power system stabilizationsystem that utilizes a data center, where a charge station, aphotovoltaic power generating system, power distribution equipment forcontrolling system stabilization, and others are located geographicallydispersedly. These facilities are coupled to the data center via anetwork, and the data center collects data from customer-side bases ofthe respective facilities and performs information processing on thecollected data. In a system as this, a redundant network is provided inorder to enhance the reliability of communication networks, and a rightdetermination about whether to correct time needs to be made also whenthe network to use is switched. This invention is therefore optimum forthis and similar systems.

In addition, the management node 402 in this invention determines foreach time slave 13 the propriety of the threshold based on actual valuesthat are related to a time packet received by the time slave 13, andchanges the threshold when the threshold is not appropriate.

This guarantees the propriety of the threshold of the time slave 13 sothat an accurate time correction can be executed.

This invention is not limited to the embodiments described above andencompasses various modification examples. For instance, the embodimentsgiven above are detailed descriptions intended for easier understandingof this invention, and this invention is not limited to a mode that hasall of the components described above. To give another example, whileone time slave is provided in one base in the time synchronizationsystems according to the embodiments described above, it should beunderstood that this invention is applicable also to a configuration inwhich one base is provided with a plurality of time slaves.

Some of or all of the components, functions, processing parts,processing measures, and the like described above may be implemented byhardware by, for example, designing those in the form of an integratedcircuit. Alternatively, the components, functions, and the likedescribed above may be implemented by software by having a processorinterpret and execute programs that respectively implement theabove-mentioned functions. Programs, tables, files, and other types ofinformation for implementing the above-mentioned functions can be put ina memory, a recording device such as a hard disk or a solid state drive(SSD), or a recording medium such as an IC card, an SD card, or a DVD.

1. A time synchronization system, comprising a time master, a timeslave, and a management node which are coupled to one another via aplurality of networks, wherein the time master transmits to the timeslave a time packet at a given timing through at least one of theplurality of networks, wherein the time slave stores thresholdinformation in which a threshold is registered for each of the pluralityof networks, wherein the time slave has: a time correction executionfeasibility determining part for receiving, through the at least one ofthe plurality of networks, the time packet transmitted from the timemaster and for determining, in reference to the threshold information,whether to execute time correction based on the received time packet;and a time correcting part for correcting, in a case where the timecorrection execution feasibility determining part determines that thetime correction is to be executed, time based on the received timepacket, and wherein the management node has: a network switchingdetermining part for determining whether to switch the network fortransmitting a time packet to another of the plurality of networks; anda network switching notifying part for notifying, in a case where it isdetermined that the network for transmitting the time packet is to beswitched, information for identifying the switched-to network to thetime slave.
 2. (canceled)
 3. The time synchronization system accordingto claim 1, wherein the management node has a threshold changedetermining part for determining whether to change the threshold storedin the time slave, wherein the time slave stores an operation valuewhich is a value related to a time packet received through the at leastone of the plurality of networks, and which is used for thedetermination by the threshold change determining part, and wherein thethreshold change determining part obtains the operation value anddetermines, based on the obtained operation value, whether to change thethreshold.
 4. The time synchronization system according to claim 3,wherein the time slave is configured to: determine whether or not theoperation value meets a given condition; and notify, in a case where itis determined that the operation value meets the given condition, thethreshold change determining part of the fact, and wherein, in a case ofbeing notified of the fact that the operation value meets the givencondition, the threshold change determining part obtains the operationvalue and determines, based on the obtained operation value, whether tochange the threshold.
 5. The time synchronization system according toclaim 3, wherein the time slave has a delay calculating part forcalculating a value about a network delay of the received time packet,and wherein the time slave stores, as the operation value, the valueabout a network delay of the received time packet that is calculated bythe delay calculating part.
 6. The time synchronization system accordingto claim 3, wherein the time slave stores, as the operation value, acount that indicates how many times the time correction executionfeasibility determining part has determined that time correction is notto be executed.
 7. The time synchronization system according to claim 1,wherein the time slave has a delay calculating part for calculating avalue about a network delay of the received time packet, and wherein thetime correction execution feasibility determining part is configured to:read a threshold that is associated with the network through which thetime packet has been received out of the threshold information; anddetermine whether or not the time correction is to be executed based onthe value about a network delay that is calculated by the delaycalculating part and the read threshold.
 8. The time synchronizationsystem according to claim 7, wherein the threshold information includesinformation for identifying, out of the plurality of networks, a networkthrough which the time master transmits the time packet, and wherein, ina case of being notified of the information for identifying theswitched-to network, the time slave updates the threshold information sothat the switched-to network is indicated as the network through whichthe time master transmits the time packet.
 9. The time synchronizationsystem according to claim 7, wherein the time master is capable oftransmitting the time packet through a plurality of networks, andwherein the time correction execution feasibility determining part isconfigured to: determine, in a case where the value about a networkdelay that is calculated by the delay calculating part is within theread threshold, whether or not the value about a network delay that iscalculated by the delay calculating part is a smallest of values about anetwork delay of the plurality of networks through which the time masteris capable of transmitting the time packet; and determine that the timecorrection is to be executed in a case where it is determined that thevalue about the network delay that is calculated by the delaycalculating part is the smallest of values about the network delay ofthe plurality of networks through which the time master is capable oftransmitting the time packet.
 10. The time synchronization systemaccording to claim 1, further comprising a network apparatus which isprovided between the time slave and the plurality of networks to relaythe time packet from the plurality of networks to the time slave,wherein the network apparatus has a plurality of time packet receivingparts for receiving the time packet from the plurality of networks in amanner that provides one time packet receiving part for each of theplurality of networks, and has a time packet transmitting part fortransmitting the received time packet to the time slave, wherein each ofthe plurality of packet receiving parts includes its own identifier inthe time packet in a case of receiving the time packet, and wherein thetime packet transmitting part transmits the time packet that includesthe identifier of the each of the plurality of packet receiving parts tothe time slave.
 11. A management node for managing a plurality ofnetworks in a time synchronization system, the system having a timemaster and a time slave which are coupled to one another via theplurality of networks, the time slave being configured to receive a timepacket transmitted from the time master at a given timing through one ofthe plurality of networks through which the time packet has beentransmitted, and to correct time based on the received time packet, themanagement node comprising: a network switching determining part fordetermining whether to switch the network for transmitting a time packetto another of the plurality of networks; a network switching notifyingpart for notifying, in a case where it is determined that the networkfor transmitting the time packet is to be switched, information foridentifying the switched-to network to the time slave; and a thresholdchange determining part for determining whether to change a thresholdused to determine whether or not time correction is to be executed bythe time slave, wherein the threshold change determining part isconfigured to: obtain from the time slave an operation value which is avalue related to a time packet received by the time slave through theone of the plurality of networks, and which is used for thedetermination by the threshold change determining part; and determinewhether to change the threshold based on the obtained operation value.12. (canceled)
 13. The management node according to claim 1, theoperation value obtained by the threshold change determining part is avalue about a network delay of the time packet received by the timeslave.
 14. The management node according to claim 1, the operation valuetable obtained by the threshold change determining part is a count thatindicates how many times the time slave has determined that timecorrection is not to be executed.
 15. A time synchronization method tobe performed in a time synchronization system, the time synchronizationsystem having a time master, a time slave, and a management node whichare coupled to one another via a plurality of networks, the time slavestoring threshold information in which a threshold is registered foreach of the plurality of networks, the time synchronization methodincluding: transmitting, by the time master, to the time slave a timepacket at a given timing through at least one of the plurality ofnetworks; receiving, by the time slave, through the at least one of theplurality of networks, the time packet transmitted from the time masterand determining, in reference to the threshold information, whether toexecute time correction based on the received time packet; correcting,in a case where it is determined that the time correction is to beexecuted, time based on the received time packet; determining, by themanagement node, whether to switch the network for transmitting a timepacket to another of the plurality of networks; and notifying, by themanagement node, in a case where it is determined that the network fortransmitting the time packet is to be switched, information foridentifying the switched-to network to the time slave.